Method and Apparatus for Wireless Device Synchronization in a Beam-Based Communication System

ABSTRACT

Among other things, the method and apparatus examples herein provide a solution to the problems that arise regarding the need for a wireless device ( 50 ) to achieve source synchronization in a beam-based system, where the wireless communication network ( 20 ) does not define “cells” for connected-mode wireless devices ( 50 ). The problems relate to how a wireless device can re-synchronize with its source access node in a beam-based system that mainly relies on self-contained transmissions from transmission points ( 30 ) in the network ( 20 ). As one example advantage, the contemplated methods and apparatus enable wireless devices ( 50 ) to perform measurements to support mobility procedures even when no data transmissions are scheduled.

TECHNICAL FIELD

The present invention relates to communication systems, such as wirelesscommunication networks, and particularly relates to radio resourcemanagement measurements and synchronizing wireless devices in beam-basedcommunication systems.

BACKGROUND

In Long Term Evolution, LTE, a user equipment, UE, can obtainsynchronization with one or more transmission points, TPs, of a cell,such as frequency and time synchronization. In one example, timesynchronization involves symbol and frame synchronization. Threerequirements for frequency and time synchronization in an LTE systeminclude symbol and frame timing, frequency synchronization, and samplingclock synchronization. Symbol and frame timing acquisition involvesdetermining the correct symbol start position. For example, the symboland frame timing is used to set a Discrete Fourier Transform, DFT,window position. Frequency synchronization is required to reduce oreliminate the effect of frequency errors arising from a mismatch oflocal oscillators between the transmitter and receiver, as well asDoppler shift caused by any UE motion.

Signal sequences used for synchronization can encode a Cyclic Prefix,CP, length, the Physical Cell Identity, PCI, and whether the cell usesFrequency Domain Duplex, FDD, or Time Domain Duplex, TDD. Due to theseproperties, the sequences that include the PCI may allow the UE to havea clear synchronization reference in the downlink for both “RRC Idle”and “RRC Connected” procedures. In “RRC Idle”, for example,synchronization allows the UE to camp on an LTE cell and possibly accessthis cell by sending a preamble to the Physical Random Access Channel,PRACH, whose configuration has been provided via system informationassociated with the cell.

A UE can obtain frequency and time synchronization with a cell—providedby a TP—in the downlink by using a Primary Synchronization Signal, PSS,and a Secondary Synchronization Signal, SSS transmitted by the TP. ThePSS and SSS structure in time is shown in FIG. 1 for the FDD case and inFIG. 2 for TDD. The TP transmits the synchronization signalsperiodically, twice per 10 ms radio frame. This arrangement allows theUEs always to be able to synchronize with any radio frame. In an FDDcell—see FIG. 1—the PSS is always located in the last OrthogonalFrequency Division Multiplexing, OFDM, symbol of the first and eleventhtime slots of each radio frame, thus enabling the UE to acquire the slotboundary timing independently of the CP length. The SSS is located inthe symbol immediately preceding the PSS, a design choice enablingcoherent detection of the SSS relative to the PSS, based on theassumption that the channel coherence duration is significantly longerthan one OFDM symbol.

In a TDD cell—see FIG. 2—the PSS is located in the third symbol of the3rd and 13th slots, while the SSS is located three symbols earlier.Coherent detection can be used under the assumption that the channelcoherence time is significantly longer than four OFDM symbols. Theprecise position of the SSS changes depending on the length of the CPconfigured for the cell. At this stage of the cell detection process,the CP length is not known a priori to the UE, and it is, therefore,blindly detected by checking for the SSS at the two possible positions.While the PSS in a given cell is the same in every subframe in which itis transmitted, the two SSS transmissions in each radio frame change ina specific manner, thus enabling the UE to establish the position of the10 ms radio frame boundary.

In the frequency domain, the mapping of the PSS and SSS to subcarriersis shown in FIG. 3. The PSS and SSS are transmitted in the central sixResource Blocks, RBs, enabling the frequency mapping of thesynchronization signals to be invariant with respect to the systembandwidth, which can in principle vary from 6 to 110 RBs to suit channelbandwidths between around 1.4 MHz and 20 MHz. This arrangement allowsthe UE to synchronize to the network without any prior knowledge of theallocated bandwidth.

The particular sequences that are transmitted for the PSS and SSS in agiven cell are used to indicate the physical layer cell identity, PCI,to the UE. There are 504 unique PCIs in LTE, grouped into 168 groups ofthree identities. The three identities in a group would usually beassigned to cells under the control of the same eNodeB. Three PSSsequences are used to indicate the cell identity within the group, and168 SSS sequences are used to indicate the identity of the group.

A study item for the new 5G radio access technology, entitled New Radioor NR, has been started in 3GPP. This study involves the followingdesign principles: ultra-lean design in the new 5G radio accesstechnology denoted as NR; self-contained transmissions; massive usage ofbeamforming; and decoupling between Idle and Connected connectivity.

NR is envisioned to be an ultra-lean system that implies theminimization of “always-on” transmissions, aiming for an energyefficient system that can account for future developments. For example,in RAN1#84bis, the RAN1 group agreed that, regarding ultra-lean design,the NR shall strive for maximizing the amount of time and frequencyresources that can be flexibly utilized or that can be left blankwithout causing backward compatibility issues in the future, where blankresources can be used for future use. Further agreed was that NR shallstrive for minimizing transmission of always-on signals, and confiningsignals and channels for physical layer functionalities—e.g., signals,channels, signaling—within a configurable/allocable time/frequencyresource.

As mentioned for LTE, PSS and SSS are the main time/frequencysynchronization enablers. They are classified as always-on signalstransmitted twice in every 10 ms radio frame. Therefore, a lean systemshould account for the need of synchronization sequences.

In LTE, a UE relies on PSS and SSS to synchronize with a given cell andsuch signals encode the PCI. The UE derives Cell-Specific ReferenceSignals, CRS, used to perform Radio Resource Management (RRM)measurements, e.g., to support DL-based mobility, and channel estimationassociated with that same PCI.

In NR, instead of relying on cell specific signals, such as PSS/SSS andCRS, “self-contained” transmissions are envisioned. Self-containedtransmissions means that all channels contain their own synchronizationsequences. The use of self-contained transmissions could be done so thata NR TP is ultra-lean to the point of not transmitting any signal, noteven for synchronization purposes unless there is an ongoing datatransmission or one that is scheduled. In this case, the UE obtainssynchronization and decodes data in the same subframe/time slot.

The limitation brought by self-contained transmissions is that therewill be periods where the UE has no data scheduled while it moves, sothat when the UE checks PDCCH availability, or the availability of anyother channel, and their self-contained signals, the UE is not able tore-synchronize because it has poor coverage. At the same time, therewill be beams that would cover the UE much better, for example, becausethe UE got closer to another TP or access node. Therefore, the UE needsto perform some kind of radio link monitoring while it is nottransmitting data, to perform measurements, send measurement reports andeventually enable the network to trigger a mobility procedure or sometype of beam management. Otherwise, the alternative would be some kindof Radio Link Failure, RLF, declaration, followed by an attempt tore-establish the connection. That approach may significantly increasethe delay until the UE can transmit data again, especially consideringthe lean design and the contemplated low periodicity of signalstransmitted for Idle UEs.

Also, there is a common understanding that NR will consider frequencyranges up to 100 GHz. In comparison to the current frequency bandsallocated to LTE, some of the new bands will have much more challengingpropagation properties, such as lower diffraction and higheroutdoor/indoor penetration losses. Consequently, signals will have lessability to propagate around corners and penetrate walls. Also, inhigh-frequency bands, atmospheric/rain attenuation and higher bodylosses render the coverage of NR signals even spottier. Fortunately, theoperation in higher frequencies makes it possible to use smaller antennaelements, which enables antenna arrays with many antenna elements. Suchantenna arrays facilitate beamforming, where multiple antenna elementsare used to form narrow beams and thereby compensate for the challengingpropagation properties. For these reasons, it is widely accepted that NRwill massively rely on antenna beamforming to provide coverage, whichmay cause some to call it a beam-based system. For example, a NR TPperforms antenna beamforming to form directional beams havingcorresponding, possibly overlapping coverage areas.

In addition, different antenna architectures should be supported,including analog, hybrid and digital. Such support implies somelimitations regarding how many directions can be covered simultaneously,especially in the case of analog/hybrid beamforming. To find a good beamdirection at a given TP, also referred to as a Transmission ReceptionPoint or TRP, access node or antenna array, a beam-sweep procedure istypically employed. A typical example of a beam-sweep procedure involvesthe node pointing a beam containing a synchronization signal and/or abeam identification signal, in each possible direction, one or a fewdirections at a time. FIG. 4 illustrates an example of beam sweeping.

A NR Cell to be discovered and used by Idle UEs may be defined by a CellIdentifier, e.g., PCI, possibly encoded by a set of synchronizationsequences like a PSS and a SSS from which the UE gets synchronization.Based on the Cell Identifier, the UE is able to obtain systeminformation and learns how to access the system. Note: Idle in thiscontext refers to the RRC Idle state but the concept extends to any kindof sleeping state where the UE is optimized for battery savings. In LTEfor example, Idle comprises procedures such as Suspend/Resume. Earlydiscussions about the NR state model referred to a new state called “RRCConnected Inactive” and that term has found some usage.

However, it is recognized and appreciated herein that a NR Cell may notneed to be defined for Connected mode UEs. Instead, the UEs may switchacross multiple beams, and Cell Identifiers are not derived frompreviously acquired information, such as the Cell ID in LTE. Such anapproach directly impacts synchronization procedures.

It is recognized herein that source synchronization is not defined for aUE in a beam-based system where “cells” as traditionally understood arenot used, at least for UEs in connected mode. This problem relates tohow the UE can re-synchronize with its source access node/TP in abeam-based system, mainly relying on self-contained transmissions. TheUE should be able to perform measurements to support mobility procedureseven when no data is scheduled.

SUMMARY

Among other things, the method and apparatus examples herein provide asolution to the problems that arise regarding the need for a wirelessdevice to achieve source synchronization in a beam-based system wherethe communication network does not define “cells” for connected-modewireless devices. The problems relate to how a wireless device canre-synchronize with its source access node in a beam-based system thatmainly relies on self-contained transmissions. As one example advantage,the contemplated methods and apparatus enable wireless devices toperform measurements to support mobility procedures even when no datatransmissions are scheduled.

In one embodiment, an access point, also referred to as a transmissionpoint, transmits one or more sets of synchronization sequences to beused by wireless devices as their synchronization source, e.g., for timeand frequency synchronization in the downlink. In a correspondingembodiment, a wireless device operating within the relevant coveragearea(s) uses any of these sequences as its synchronization source. Thesynchronization signals can be beamformed in narrow beams or widerbeams.

As an advantage of such operation, the wireless device can maintain itssynchronization when moving across the coverage areas of differentbeams, where different synchronization signals are being transmitted foreach beam. For example, the synchronization signal transmitted for eachbeam is based on a different sequence.

This disclosure also presents method and apparatus details for wirelessdevices and the network to obtain or determine the synchronizationsequences in use by respective transmission points, and for updatingsuch information to support mobility, beam configuration changes, etc.

Further, in at least one embodiment, the different synchronizationsignals transmitted by a transmission point are different MobilityReference Signals (MRS). Each MRS comprises a Time SynchronizationSequence (TSS) and Beam Reference Signal (BRS). Thus, a transmissionpoint is configured with a set of MRSs, with each MRS corresponding to abeam, such that the set of MRSs transmitted by the transmission pointenables a wireless device to use the transmission point as asynchronization source.

The availability of multiple sets of synchronization sequences/MRSsallows wireless devices to re-gain synchronization even when the devicecannot decode downlink control channels, or suffers from other types ofradio link problems.

With the above in mind, in one or more embodiments, a transmission pointis configured for operation in a wireless communication network. Thetransmission point comprises transceiver circuitry and associatedprocessing circuitry operative to transmit two or more synchronizationsignals from the transmission point. Respective ones of thesynchronization signals correspond to different beams from among two ormore beams used by the transmission point in antenna beamforming. In anexample, the transmission point is configured to use a set ofsynchronization sequences, where it will be understood that eachsynchronization signal is based on a different one of the sequences. Thesynchronization signals serve as references for synchronizationmeasurements by wireless devices, for obtaining or maintainingsynchronization with the transmission point.

In at least some embodiments, a wireless device is configured foroperation in a wireless communication network. The wireless devicecomprises transceiver circuitry and operatively associated processingcircuitry configured to determine the synchronization signals used by atransmission point of the wireless communication network. Moreparticularly, the synchronization signals comprise two or moresynchronization signals, and the transmission point uses a differentsynchronization signal for each of two or more beams used by thetransmission point in antenna beamforming. The wireless devicesynchronizes to the transmission point based on receiving one or more ofthe synchronization signals transmitted by the transmission point.

Further aspects of the present invention are directed to an apparatus,computer program products or computer readable storage mediumcorresponding to the methods summarized above and functionalimplementations of the above-summarized apparatus and wireless device.

Of course, the present invention is not limited to the above featuresand advantages. Those of ordinary skill in the art will recognizeadditional features and advantages upon reading the following detaileddescription, and upon viewing the accompanying drawings.

In one embodiment a method implemented by a transmission pointconfigured for operation in a wireless communication network isprovided. The method comprises transmitting two or more synchronizationsignals from the transmission point on a beam-specific basis, such thateach synchronization signal corresponds to a respective one among two ormore beams used by the transmission point in antenna beamforming. Thesynchronization signals serve as references for synchronizationmeasurements by wireless devices, for obtaining or maintainingsynchronization with the transmission point. This provides the advantagethat a wireless device can maintain synchronisation with a transmissionpoint whilst it moves across directional beams transmitted by thetransmission point by detecting alternative synchronisation signals towhich it can use for synchronisation as it moves into the path of thebeam.

In a further aspect, the method comprises transmitting information fromthe transmission point to enable the wireless device to determine thetwo or more synchronization signals as belonging to a set ofsynchronisation signals.

In another aspect the method includes transmitting downlink signalsusing the two or more directional beams.

In another aspect the method includes generating the two or moresynchronization signals wherein the generating includes differentiatingthe two or more synchronization signals in terms of included informationor signal properties, thereby enabling a receiving wireless device todistinguish between the two or more synchronization signals.

In some aspects, the transmission point generates two or moresynchronization signals, each synchronization signal beingdistinguishable from the other one or ones of the two or moresynchronization signals.

In another aspect the two or more synchronization signals comprise a setof Mobility Reference Signals, MRSs, each MRS comprising a TimeSynchronization Signal, TSS, and a Beam Reference Signal, BRS, and eachBRS being unique within the set of MRSs, and further wherein each MRS isassociated with a different one of the two or more directional beamsused by the transmission point in antenna beamforming.

In another aspect, the method further comprises transmitting the two ormore synchronization signals according to a beam-sweeping pattern.

In another aspect, the method further comprises adapting thetransmission of at least one synchronization signal for thecorresponding directional beam in dependence on at least one of: radiolink conditions between the transmission point and one or more wirelessdevices that are operating in a coverage area of the correspondingdirectional beam; and a monitored synchronization quality of one or morewireless devices operating in the coverage area of the correspondingdirectional beam.

In another aspect the method further comprises receiving signaling fromanother node in the wireless communication network, specifying the twoor more synchronization signals to be generated and transmitted.

In another aspect the method further comprises, for one or more of thedirectional beams, dynamically determining at the transmission pointwhich downlink resources to use for transmitting the correspondingsynchronization signal.

In another aspect, the method further comprises transmitting assistanceinformation from the transmission point, the assistance informationidentifying the two or more synchronization signals, or otherwiseproviding information needed for wireless devices to detect or identifythe two or more synchronization signals.

In another aspect the method further comprises dynamically changing thenumber of directional beams used by the transmission point in antennabeamforming and correspondingly changing the number of synchronizationsignals in use by the transmission point, such that the transmissionpoint transmits a different synchronization signal for each directionalbeam.

In another embodiment a transmission point is configured for operationin a wireless communication network, the transmission point comprisingtransceiver circuitry and processing circuitry operative to transmit,via the transceiver circuitry, two or more synchronization signals fromthe transmission point on a beam-specific basis, such that eachsynchronization signal corresponds to a respective one among two or moredirectional beams used by the transmission point in antenna beamforming.The synchronization signals serve as references for synchronizationmeasurements by wireless devices, for obtaining or maintainingsynchronization with the transmission point.

In another embodiment a method of operation by a wireless deviceconfigured for operation in a wireless communication network isprovided. The method comprises determining a set of synchronizationsignals used by a transmission point in the wireless communicationnetwork and maintaining synchronization with the transmission point inconjunction with moving between coverage areas corresponding to the twoor more directional beams, based on detected ones of the synchronizationsignals in the set. This provides the advantage that the wireless deviceavoids performing new synchronization procedures with the transmissionpoint when it enters the coverage of a new beam, since the beams aredetermined to belong to the same transmission point.

In another aspect the determining the set of synchronization signalscomprises receiving assistance information that identifies the set ofsynchronization signals, or provides information enabling the wirelessdevice to identify the set of synchronization signals.

In another aspect the transmission point comprises a first one ofneighboring first and second transmission points in the wirelesscommunication network. The method further comprises detecting one ormore synchronization signals comprising a set determined to beassociated with the second transmission point, and changing over fromusing the first transmission point as the synchronization source for thewireless device to using the second transmission point as thesynchronization source for the wireless device, based on determiningthat a radio quality determined by the wireless device for one or moresynchronization signals detected from the second transmission point ishigher than a radio quality determined by the wireless device for anydetected synchronization signal from the first transmission point.

In another aspect, the method further comprises determining, based onreceiving information from the wireless communication network, thedownlink resources used for transmitting each synchronization signal forthe corresponding directional beam.

In another aspect, maintaining synchronization with the transmissionpoint comprises, in instances where two or more of the synchronizationsignals in the set of synchronization signals are detected by thewireless device, selecting a strongest or highest-quality one of the twoor more detected synchronization signals, for use in maintainingsynchronization with the transmission point.

In another embodiment, a wireless device is configured for operation ina wireless communication network. The wireless device (50) comprisestransceiver circuitry for receiving signals from transmission points(30) in the wireless communication network and processing circuitryoperatively associated with the transceiver circuitry and configured todetermine a set of synchronization signals used by a transmission pointand maintain synchronization with the transmission point in conjunctionwith moving between coverage areas corresponding to the two or moredirectional beams, based on dynamically synchronizing or resynchronizingwith detected ones of the synchronization signals in the set.

In another embodiment a computer program product comprises computerinstructions that, when executed on at least one processing circuit of atransmission point configured for operation in a wireless communicationnetwork, cause the transmission point to transmit two or moresynchronization signals from the transmission point on a beam-specificbasis, such that each synchronization signal corresponds to a respectiveone among two or more directional beams used by the transmission point(30) in antenna beamforming. The synchronization signals serve asreferences for synchronization measurements by wireless devices, forobtaining or maintaining synchronization with the transmission point.

In another embodiment a computer program product comprises computerinstructions that, when executed on at least one processing circuit of awireless device configured for operation in a wireless communicationnetwork, cause the wireless device to determine a set of synchronizationsignals used by a transmission point and maintain synchronization withthe wireless communication network in conjunction with moving betweencoverage areas corresponding to the two or more directional beams, basedon dynamically synchronizing or resynchronizing with detected ones ofthe synchronization signals in the set.

In another embodiment a transmission point is configured for operationin a wireless communication network and comprises a transmitting moduleand a processing module configured to transmit, via the transmittingmodule, the two or more synchronization signals from the transmissionpoint on a beam-specific basis, such that each synchronization signalcorresponds to a respective one among two or more directional beams usedby the transmission point in antenna beamformin. The synchronizationsignals serve as references for synchronization measurements by wirelessdevices, for obtaining or maintaining synchronization with thetransmission point.

In another embodiment a wireless device is configured for operation in awireless communication network and comprising a receiving moduleconfigured to receive signals from a transmission point in the wirelesscommunication network and a processing module configured to determine aset of synchronization signals used by the transmission point andmaintain synchronization with the transmission point in conjunction withmoving between coverage areas corresponding to the two or moredirectional beams, by dynamically synchronizing or resynchronizing withdetected ones of the synchronization signals in the set.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating PSS and SS frame and slot structure inthe time domain in the FDD case.

FIG. 2 is a diagram illustrating PSS and SS frame and slot structure inthe time domain in the TDD case.

FIG. 3 is a diagram illustrating PSS and SSS frame structure infrequency and time domain for an FDD cell.

FIG. 4 is a diagram illustrating a beam sweeping procedure.

FIG. 5A is a diagram illustrating an example wireless communicationnetwork, including one or more transmission points.

FIG. 5B is a diagram illustrating one embodiment of transmittingmultiple synchronization sequences from a transmission point, for use bya wireless device.

FIG. 6 is a block diagram of a network node, according to someembodiments.

FIG. 7 illustrates a method of operation in a wireless communicationnetwork, according to some embodiments.

FIG. 8 is a block diagram of a user equipment, according to someembodiments.

FIG. 9 illustrates a method of operation in a wireless communicationnetwork, according to some embodiments.

FIG. 10 is a diagram illustrating a user equipment that considers itselfsynchronized with the source at different times using different sets ofsequences, according to some embodiments.

FIG. 11 is a diagram illustrating various numbers of sets ofsynchronization signals to be used as a synchronization source,according to some embodiments.

FIG. 12 is a block diagram illustrating a functional implementation of anetwork node, according to some embodiments.

FIG. 13 is a block diagram illustrating a functional implementation of awireless device, according to some embodiments.

DETAILED DESCRIPTION

It is recognized and appreciated herein that by having the same coverageproperties of a synchronization signal and a beam identification signal,a wireless device could not only synchronize to a transmission point,but also gain the best beam knowledge at a given location.

Further, it is appreciated herein that, concerning synchronizationacquisition, the transmitted synchronization signal in the downlink (ifat all available) might happen in a beam-sweep manner, and thus awireless device might be able to hear several synchronization signalsall belonging to the same node.

In an example embodiment, a transmission point transmits two or moresynchronization signals, e.g., it transmits a set of synchronizationsignals. Here, the transmission point comprises, e.g., a network accessnode. Further, transmitting two or more synchronization signals shouldbe understood as referring in a general sense to the transmission of twoor more synchronization signals that are distinguishable, e.g., onebased on a first sequence and one based on a second sequence. It shouldalso be understood that “transmitting” the two or more synchronizationsignals may include transmitting them at multiple times, e.g., in anynumber of recurring frames or subframes. Further, transmitting the twoor more synchronization signals may mean transmitting them at the sametimes, or at different times, etc.

These synchronization signals may include synchronization sequences,such as PSS/SSS or Mobility Reference Signals, MRSs. MRSs may refer tosignals that are transmitted in a wireless network and that arespecifically designated for measurements by wireless devices, where themeasurements are for use in mobility procedures, e.g., handovers fromone node to another or from one beam to another. Sometimes, an MRS mayalso be referred to as a Measurement Reference Signal. MRSs may includea Time Synchronization Sequence, TSS, and a Beam Reference Signal, BRS.A set of synchronization signals may include a pair of sequences, suchas the TSS and the BRS. In other words, “multiple sets ofsynchronization sequences” can be interpreted as “multiple MRSs”. A setof MRSs may be used as the synchronization source for a given userequipment, UE. Note that “UE” and “wireless device” may be considered asinterchangeable terms unless otherwise noted.

An MRS set may also refer to a set of parameters that defines thephysical resources occupied by a transmitted MRS, i.e., thetime-frequency and/or code resources, and/or that defines a signalsequence, such as a sequence of symbol values that make up the MRS.Thus, for example, different MRS sets may specify differenttime-frequency resources for different MRSs, such as different patternsof resource elements in an Orthogonal Frequency-Division Multiplexing(OFDM) time-frequency grid. Different MRS sets may instead oradditionally specify different sequences of symbol values, for example.

According to various embodiments, one or more sets of signals may bereceived by wireless devices under the coverage area of a transmissionpoint, which may be an access node, and used for various purposes, suchas for synchronization or RRM measurements. For example, a set ofsignals may be a set of synchronization signals, where a differentsynchronization signal belongs to each of two or more directional beamsused by a transmission point, each directional beam covering arespective portion of a coverage area of the transmission point fordownlink service. A wireless device that receives the signal set, ormultiple signal sets, may be able to autonomously determine to use thesignal set as its synchronization source for time and frequencysynchronization in the downlink. For example, the synchronizationsignals of a received signal set can be used, among other information,to keep a wireless device synchronized when it moves across the coverageof different sets of sequences being transmitted by the sametransmission point or access node. There is no need to trigger any errorcase or failure procedure, such as Radio Link Failure, as long as any ofthese sets can be detected and synchronization can be obtained fromthem.

FIGS. 5A and 5B illustrate example arrangements contemplated herein.Particularly, FIG. 5A illustrates an example wireless communicationnetwork 20, which includes one or more supporting nodes 22—e.g.,operations nodes, gateway nodes, etc.—that provide support for severaltransmission points 30 that provide radio access to the network 20.Three transmission points 30 appear in the diagram, includingtransmission points 30-1, 30-2, and 30-3. While referred to as“transmission points”, it will be appreciated that each transmissionpoint 30 may operate as an access point, base station, etc., providingfor both downlink transmissions to wireless devices operating in thenetwork 20 and reception of uplink transmissions from such devices. Forsimplicity, only one example wireless device 50 appears in thefigure—denoted as “UE 50”—and it will be appreciated that the wirelessdevice 50 may move in or among the radio coverage areas associated withthe respective transmission points 30.

In that regard, each transmission point 30 in FIG. 5A operates as abeamforming transmission point, wherein it uses two or more directionalbeams, with each beam providing radio coverage in a correspondingcoverage area. Of course, the beam-to-beam coverage areas may overlap,and the transmission point 30 may dynamically adjust any one or more ofthe beam size, beam shape, and beam count.

FIG. 5B illustrates one example of beamforming by a transmission point30, where the transmission of multiple synchronization signals by thetransmission point 30 is to be used by a wireless device 50 as itssynchronization source for time and frequency synchronization in thedownlink. The multiple synchronization signals 110, 120, 130 and 140 canbe beamformed in narrow beams or wider beams. Provided the wirelessdevice 50 can detect any of these signals, the wireless device 50 canconsider itself synchronized with the source access node or transmissionpoint. In some cases, sets of signals can be used for update procedures,such as in the case of mobility and/or parameter optimization.

In the context of FIG. 5B, the synchronization signal 110 can beunderstood as being a first synchronization sequence from a defined setof synchronization sequences, with the synchronization signal 120 beinga second synchronization sequence from the set, and so on.

FIG. 6 illustrates an example transmission point 30 in more detail, butit should be understood that the transmission point 30 may beimplemented differently. Further, the transmission point 30 should beunderstood as being an example of a radio network node, such as anaccess point, base station, eNodeB, gNB, or another transceiver. Stillfurther, at least some of the functionality attributed to thetransmission point 30 may be distributed across more than one node,e.g., at least some functionality may be performed by other nodes in theradio network, or in an associated core network, or may be cloud-based.

The transmission point 30, also referred to as the network node 30,includes a communication interface circuit 38 that includes circuitryfor communicating with other network nodes 22. The network node 30communicates with wireless devices 50 operating in the network viaantennas 34 and transceiver circuitry 36. The antenna(s) 34 comprises,for example, an array of antenna elements and the transceiver circuitry36 is configured to perform beamforming using the antenna array.

Broadly, the transceiver circuitry 36 may include transmitter circuitry,receiver circuitry, and associated control circuits that arecollectively configured to transmit and receive signals according to oneor more radio access technologies, for the purposes of providingcommunication services, at least to wireless devices 50 operating in thecoverage area(s) associated with the network node 30. For example, thenetwork node 30 is configured as an NR node providing radio access in anNR network.

The network node 30 also includes processing circuitry 32 that isoperatively associated with the communication interface circuit 38 andtransceiver circuitry 36. The network node 30 uses the communicationinterface circuit 38 to communicate with other network nodes 22 and thetransceiver circuit 36 to communicate with wireless devices 50. By wayof example, the processing circuitry 32 comprises one or more digitalprocessors 42, e.g., one or more microprocessors, microcontrollers,Digital Signal Processors (DSPs), Field Programmable Gate Arrays(FPGAs), Complex Programmable Logic Devices (CPLDs), ApplicationSpecific Integrated Circuits (ASICs), or any mix thereof. Moregenerally, the processing circuitry 32 may comprise fixed circuitry, orprogrammable circuitry that is specially configured via the execution ofprogram instructions implementing the functionality taught herein, ormay comprise some mix of fixed and programmed circuitry.

In the illustrated embodiment, the processing circuitry 32 includes amemory 44. The memory 44, in some embodiments, stores one or morecomputer programs 46 and, optionally, configuration data 48. The memory44 provides non-transitory storage for the computer program 46, and itmay comprise one or more types of computer-readable media, such as diskstorage, solid-state memory storage, or any mix thereof. By way ofnon-limiting example, the memory 44 comprises any one or more of SRAM,DRAM, EEPROM, and FLASH memory, which may be in the processing circuitry32 and/or separate from the processing circuitry 32.

In general, the memory 44 comprises one or more types ofcomputer-readable storage media providing non-transitory storage of thecomputer program 46 and any configuration data 48 used by the networknode 30. Here, “non-transitory” means permanent, semi-permanent, or atleast temporarily persistent storage and encompasses both long-termstorage in non-volatile memory and storage in working memory, e.g., forprogram execution.

In some embodiments, the processor(s) 42 of the processing circuitry 32may execute a computer program 46 stored in the memory 44 thatconfigures the processor(s) 42 to control the transmitting circuitry ofthe transceiver circuit 36 to transmit a different synchronizationsignal for each of two or more beams used by the network node 30 inantenna beamforming. Each such beam covers, for example, a respectiveportion of the overall coverage area of the network node 30. Thesynchronization signal transmitted for each directional beam belongs toa set of synchronization signals associated with the network node 30 andenables wireless devices 50 operating in the coverage area(s) of thenetwork node 30 to synchronize with the wireless communication network20 via the network node 30. The synchronization signals also may haveproperties that allow them to be used by wireless devices 50 asreference signals for making received signal strength or quality orother radio measurements.

In some embodiments the processing circuitry 32 is configured totransmit, via the transceiver circuitry 36, two or more synchronizationsignals from the network node 30 on a beam-specific basis, such thateach synchronization signal corresponds to a respective one among two ormore directional beams used by the transmission point in antennabeamforming. The synchronization signals serve as references forsynchronization measurements by wireless devices 50, for obtaining ormaintaining synchronization with the network node 30.

In some embodiments the processing circuitry 32 of the network node 30is configured to generate two or more synchronization signals, eachsynchronization signal being distinguishable from the other one or onesof the two or more synchronization signals. Further, the processingcircuitry 32 is configured to transmit, via the transceiver circuitry36, the two or more synchronization signals from the network node 30 ona beam-specific basis, such that each synchronization signal correspondsto a respective one among two or more directional beams used by thetransmission point in antenna beamforming. The synchronization signalsserve as references for synchronization measurements by wireless devices50, for obtaining or maintaining synchronization with the network node30.

In some embodiments the processing circuitry 32 of the network node 30is configured to generate two or more synchronization signals, whereineach synchronization signal belongs to a set of synchronization signalsassociated with the transmission point. Further, the processingcircuitry 32 is configured to transmit, via the transceiver circuitry36, the two or more synchronization signals from the network node 30 ona beam-specific basis, such that each synchronization signal correspondsto a respective one among two or more directional beams used by thetransmission point in antenna beamforming. The synchronization signalsserve as references for synchronization measurements by wireless devices50, for obtaining or maintaining synchronization with the network node30. In further example details, the processing circuitry 32 isconfigured to transmit downlink signals using the two or moredirectional beams. In the same or in other embodiments, the processingcircuitry 32 is configured to differentiate the two or moresynchronization signals in terms of included information or signalproperties, thereby enabling a receiving wireless device 50 todistinguish between the two or more synchronization signals.

The two or more synchronization signals comprise, for example, a set ofMobility Reference Signals, MRSs. In turn, each MRS comprises a TimeSynchronization Signal, TSS, and a Beam Reference Signal, BRS, and eachBRS being unique within the set of MRSs. Further, each MRS is associatedwith a different one of the two or more directional beams used by thetransmission point in antenna beamforming.

Further, in one or more embodiments, the processing circuitry 32 isconfigured to transmit the two or more synchronization signals accordingto a beam-sweeping pattern. In the same embodiment(s), or in stillfurther embodiments, the processing circuitry 32 is configured to adaptthe transmission of at least one synchronization signal for thecorresponding directional beam in dependence on at least one of: radiolink conditions between the transmission point and one or more wirelessdevices 50 that are operating in a coverage area of the correspondingdirectional beam, a monitored synchronization quality of one or morewireless devices 50 operating in the coverage area of the correspondingdirectional beam.

The processing circuitry 32 is further configured in at least someembodiments to receive signaling from another node in the wirelesscommunication network 20—e.g., from a supporting node 22—where suchsignaling indicates the two or more synchronization signals to begenerated and transmitted. Still further, in at least some embodiments,the processing circuitry 32 is configured to dynamically determine whichdownlink resources to use for transmitting the correspondingsynchronization signal for one or more of the directional beams.

The processing circuitry 32 may also be configured to transmitassistance information from the network node 30. The assistanceinformation identifies the two or more synchronization signals, orotherwise provides information needed for wireless devices 50 to detector identify the two or more synchronization signals. Still further, theprocessing circuitry 32 may be configured to dynamically change thenumber of directional beams used by the network node 30 in antennabeamforming and correspondingly change the number of synchronizationsignals in use by the network node 30, such that the network node 30transmits a different synchronization signal for each directional beam.

More generally, a network node 30 may carry out a method or methods ofoperation embodying any of the above functionality described for thenetwork node 30, without being restricted to the implementation detailsillustrated in the example of FIG. 6. Correspondingly, FIG. 7illustrates an example method 700 performed by a network node 30.

The method 700 includes generating (Block 702) two or moresynchronization signals, each synchronization signal beingdistinguishable from the other one or ones of the two or moresynchronization signals, and transmitting (Block 704) the two or moresynchronization signals from the transmission point on a beam-specificbasis, such that each synchronization signal corresponds to a respectiveone among two or more directional beams used by the transmission pointin antenna beamforming. Here, the synchronization signals serve asreferences for synchronization measurements by wireless devices 50, forobtaining or maintaining synchronization with the transmission point 30.

The method (700) may include generating (702) two or moresynchronization signals, wherein each synchronization signal belongs toa set of synchronization signals associated with the transmission pointand transmitting (704) the two or more synchronization signals from thetransmission point (30) on a beam-specific basis, such that eachsynchronization signal corresponds to a respective one among two or moredirectional beams used by the transmission point (30) in antennabeamforming; wherein the synchronization signals serve as references forsynchronization measurements by wireless devices (50), for obtaining ormaintaining synchronization with the transmission point (30). Assuggested above, the method 700 may include further operations, such astransmitting assistance information identifying the two or moresynchronization signals, e.g., identifying the set of synchronizationsignals being used by the network node 30 for the involved set ofdirectional beams. The method 700 may also include transmitting thesynchronization signals according to a beam-sweeping pattern. Stillfurther, the method 700 may include adapting the transmission of atleast one synchronization signal for the corresponding directional beamin dependence on at least one of: radio link conditions between thenetwork node 30 and one or more wireless devices 50 that are operatingin a coverage area of the corresponding directional beam, a monitoredsynchronization quality of one or more wireless devices 50 operating inthe coverage area of the corresponding directional beam.

Also as noted, the two or more synchronization signals may comprise aset of Mobility Reference Signals, MRSs. As before, each MRS maycomprise a Time Synchronization Signal (TSS) and a Beam ReferenceSignal, BRS. Each BRS is unique within the set of MRSs, and each MRS isassociated with a different beam used by the transmission point 30 forantenna beamforming.

Further, there may be beamforming solutions where different sets ofbeams are used to provide coverage for an overall coverage area. Forexample, there could be beam widths of different granularity within thesame overall coverage area. A particular set of beams can be used atdifferent times depending on what type of wireless devices 50 are in theoverall coverage area. Any presence of slow moving distant wirelessdevices 50 might involve a configuration with a couple of narrow beamsas a fall back synchronization source for such devices. If there aremany wireless devices 50 in the coverage area, then the network node30—or multiple, coordinating nodes 30—might be configured to provide thewireless devices 50 with a few common wide area beams as synchronizationsources. Basically, the involved beamformers—network node or nodes30—may be configured with the flexibility to determine what type ofbeamforming is used to provide synchronization coverage. For instance,at different times, different beams might be providing the coverage inan overall coverage area, depending on the current population of theactive wireless devices in the overall coverage area.

The multiple, beam-specific synchronization signals transmitted by agiven network node 30 may be distinguished by wireless device 50 basedon, for example different identification information being included inthe synchronization signals. In further refinements, synchronizationsignals may be device-specific, or specific to a group of wirelessdevices 50.

FIG. 8 illustrates a diagram of an example wireless device 50 that isconfigured according to the teachings herein. The wireless device 50comprises essentially any type of device or apparatus having wirelesscommunication capability and configured for operation in a wirelesscommunication network 20 of the types at issue in this disclosure.

The wireless device 50 communicates with a radio node or base station,such as the network access node 30, via antennas 54 and transceivercircuitry 56. The transceiver circuitry 56 may include transmittercircuitry, receiver circuitry, and associated control circuits that arecollectively configured to transmit and receive signals according to oneor more radio access technologies (RATs).

The wireless device 50 also includes processing circuitry 52 that isoperatively associated with the transceiver circuitry 56. In one or moreembodiments, the processing circuitry 52 comprises one or more digitalprocessing circuits, e.g., one or more microprocessors,microcontrollers, DSPs, FPGAs, CPLDs, ASICs, or any mix thereof. Moregenerally, the processing circuitry 52 may comprise fixed circuitry, orprogrammable circuitry that is specially adapted via the execution ofprogram instructions implementing the functionality taught herein, ormay comprise some mix of fixed and programmed circuitry. The processingcircuitry 52 may be multi-core.

The processing circuitry 52 in the example embodiment also includes oris associated with a memory 64. The memory 64, in some embodiments,stores one or more computer programs 66 and, optionally, configurationdata 68. The memory 64 provides non-transitory storage for the computerprogram 66, and it may comprise one or more types of computer-readablemedia, such as disk storage, solid-state memory storage, or any mixthereof. By way of non-limiting example, the memory 64 comprises any oneor more of SRAM, DRAM, EEPROM, and FLASH memory, which may be in theprocessing circuitry 52 and/or separate from processing circuitry 52. Ingeneral, the memory 64 comprises one or more types of computer-readablestorage media providing non-transitory storage of the computer program66 and any configuration data 68 used by the wireless device 50.

In some embodiments, the processor 62 of the processing circuitry 52 mayexecute a computer program 66 stored in the memory 64 that configuresthe processor 62 to operate as detailed herein.

With FIG. 8 in mind as a non-limiting example, a wireless device 50 isconfigured for operation in a wireless communication network 20. Theexample wireless device 50 includes transceiver circuitry 56 forreceiving signals from transmission points 30 in the wirelesscommunication network 20, and processing circuitry 52 that isoperatively associated with the transceiver circuitry 56.

The processing circuitry 52 is configured to determine a set ofsynchronization signals used by a transmission point 30. Eachsynchronization signal in the set is associated with a differentdirectional beam used by the transmission point 30 in antennabeamforming, and the transmission point 30 uses two or more directionalbeams.

Further, the processing circuitry 52 is configured to maintainsynchronization of the wireless device 50 with the transmission point 30in conjunction with moving between coverage areas corresponding to thetwo or more directional beams, based on dynamically synchronizing orresynchronizing with detected ones of the synchronization signals in theset. In the same or at least one further embodiment, in instances wheretwo or more of the synchronization signals in the set of synchronizationsignals are detected, the processing circuitry 52 is configured toselect a strongest or highest-quality one of the two or more detectedsynchronization signals, for synchronization. In this way the wirelessdevice may autonomously maintain synchronization with the transmissionpoint 30.

In at least some embodiments, the processing circuitry 52 is configuredto determine the set of synchronization signals based on receivingassistance information that identifies the set of synchronizationsignals, or provides information enabling the wireless device 50 toidentify the set of synchronization signals. For example, thetransmission point 30 transmits the assistance information, which isreceived by the processing circuitry 52 via the transceiver 56.

In an example scenario, the transmission point 30 comprises a first oneof neighboring first and second transmission points 30-1 and 30-2 in thewireless communication network 20. Correspondingly, the processingcircuitry 52 is configured to detect one or more synchronization signalscomprising a set of synchronization signals associated with the secondtransmission point 30-2, and change over from using the firsttransmission point 30-1 as the synchronization source for the wirelessdevice 50 to using the second transmission point 30-2 as thesynchronization source for the wireless device 50-2, based ondetermining that a radio quality determined by the wireless device 50for one or more synchronization signals detected from the secondtransmission point 30-2 is higher than a radio quality determined by thewireless device 50 for any detected synchronization signal from thefirst transmission point 30-1. In this way the the update of the set ofsynchronization signals to be considered as a synchronization source canbe done autonomously by the wireless device

Still further, in one or more embodiments, the processing circuitry 52is configured to determine, based on receiving information from thewireless communication network 20, the downlink resources used fortransmitting each synchronization signal for the correspondingdirectional beam. Also, as noted, the set of synchronization signals maycomprise a set of Mobility Reference Signals, MRSs, each MRS comprisinga Time Synchronization Signal, TSS, and a Beam Reference Signal, BRS,and each BRS being unique within the set of MRSs, and further whereineach MRS is associated with a different one of the two or moredirectional beams used by the transmission point in antenna beamforming.

FIG. 9 illustrates one embodiment of a method 900 of processingimplemented by a wireless device 50, such as the example wireless device50 depicted in FIG. 8. However, the method 800 may be carried out bywireless devices having circuitry arrangements different than thoseillustrated in FIG. 8.

The method 900 includes the wireless device 50 determining (Block 902) aset of synchronization signals used by a transmission point 30. In someembodiments, each synchronization signal in the set may be associatedwith a different directional beam used by the transmission point 30 inantenna beamforming, and where the transmission point 30 uses two ormore directional beams.

The method 900 further includes the wireless device 50 maintainingsynchronization with the transmission point 30 in conjunction withmoving between coverage areas corresponding to the two or moredirectional beams. Such operations include dynamically synchronizing orresynchronizing with detected ones of the synchronization signals in theset. The method 900 may further include, in instances where two or moreof the synchronization signals in the set of synchronization signals aredetected, selecting a strongest or highest-quality one of the two ormore detected synchronization signals, for synchronization. In this waythe wireless device may autonomously maintain synchronization with thetransmission point 30.

The method 900 may further include determining at least one of thefollowing, based on the wireless device 50 receiving information fromthe wireless communication network 20: the set of synchronizationsignals, and the particular downlink resources used for transmittingeach synchronization signal for the corresponding directional beams. Forexample, the wireless device 50 updates or changes the set ofsynchronization signals that it attempts to detect for synchronization,based on at least one of an autonomous decision by the wireless device50 and initiation by the wireless communication network.

FIG. 10 is a diagram illustrating a wireless device 50 that considersitself synchronized with the source at different times (t0, t1, and tk)using different sets of sequences 110, 120, 130 and 140. These multiplesets of sequences 110-140 can be beamformed in different directions. Thesets of sequences, for example in the case of analog/hybrid beamforming,can rely on a beam sweeping procedure to be possibly detected bywireless devices 50 in multiple directions.

In the example of FIG. 10, the wireless device 50 determines that itwill use a first synchronization sequence 110 at time t0. The wirelessdevice 50 later determines that it will use a second synchronizationsequence 120 at time t1 and a third synchronization sequence 140 at timetk.

The number of sets of synchronization sequences (i.e., MRSs) may varyfrom one access node to another in order to enable the network toconfigure beam sweeping differently (periodicity, repetition factor ofeach MRS per sweep, number of MRSs per sweep, etc.) for different accessnodes or transmission points and/or network deployments. Therefore, insome cases, the wireless device 50 may use two sets while in other casesit may use three sets or more.

Accordingly, FIG. 11 is a diagram illustrating that the number ofsynchronization signals to be used as a synchronization source can varyfrom one network node 30 to another. The network node 30-1 transmits twosynchronization signals 110 and 120, e.g., one in each beam used by thenetwork node 30-1 in antenna beamforming. However, the network node 30-2transmits three synchronization signals 140, 150, 160, e.g., one in eachbeam used by the network node 30-2 in antenna beamforming. Thus, thewireless device 50-1 may use one or both of the synchronization signals110 and 120 as its synchronization source, while the wireless device50-2 may use any one or more of the synchronization signals 140, 150,and 160 as its synchronization source. These multiple synchronizationsignals can be transmitted periodically per beam sweeping cycle or basedon other triggering criteria identified by involved nodes 30, or asupporting node 22 in the network 20, e.g., based on the uplink quality.

In some embodiments, the synchronization signals are configured eitheras periodic signals, for that source synchronization purpose, or asaperiodic signals so that the network 20 detects when wireless devices50 need to re-synchronize with their source nodes 30.

In some embodiments, a wireless device 50 receives information from thenetwork 20 indicating the synchronization signals that it should use assynchronization sources—e.g., it receives information indicating one ormore sets of synchronization signals used by one or more transmissionpoints 30 in the network 20. Such information is helpful because thewireless device 50 may autonomously detect multiple synchronizationsignals but not know the associations between synchronization signalsand corresponding beams, nor necessarily know which synchronizationsignals are preferred.

Consider a transmission point 30 that beamforms according to a given setof beams, with each beam associated with a respective synchronizationsignal in a corresponding set of synchronization signals. Indicating theset of synchronization signals to the wireless device 50 allows it torecognize any of the beams in the beamforming set, and to maintainsynchronization with the transmission point 30 as it moves within thecoverage areas of the respective beams. Extending this example, thewireless device 50 could be in the coverage areas of multipletransmission points 30, each of them using respective sets of beams andcorresponding sets of synchronization signals, e.g., MRSs, for beamdifferentiation. Of course, a given transmission point 30 may usemultiple sets of beams, with each beam set having a corresponding set ofsynchronization signals. Further, a transmission point 30 may indicate asubset or restricted set of synchronization signals to be used by awireless device 50 for obtaining or maintaining synchronization with thetransmission point 30. Any or all such information may be regarded as“assistance” information that may be signaled or otherwise indicated towireless devices 50 operating in the network 20.

The wireless device 50 may obtain such information from systeminformation broadcasted within the same coverage area but notnecessarily by the transmission point 30 that is or will serve as thesynchronization source for the wireless device 50. This operation mayoccur before initial access or when the wireless device 50 is in Idlemode, such as before a transition to Connected mode. Alternatively, thewireless device 50 may be explicitly configured with the particularsynchronization signals, e.g., set or sets, to be used, via dedicatedsignaling. This approach presumes that the wireless device 50 obtainsits source synchronization set once it is in Connected mode.

In other cases, the wireless device 50 derives, deduces, or otherwiseinfers the synchronization signals to be used for sourcesynchronization, based on detecting an identifier, such as a Cell ID,System Information Index (SS), System Signature (SS), etc. In thesecases, the mapping between the ID and the synchronization signals to beused could either be defined in the standards or otherwise obtained bythe wireless device 50, e.g., based on such information beingtransmitted in an Access Information Table, AIT. Note that the “mapping”in such embodiments may be a mapping between IDs andsynchronization-signal sequences, or sets of sequences.

The synchronization signals can be transmitted either in a fixed part ofthe spectrum, e.g., in the central N resource blocks of a givenfrequency band, or in a more flexible manner, e.g., in one of thepossible parts of the band. In the flexible case, the wireless device 50can derive the time-frequency resources, or where to look for the set ofsynchronization sequences, from another identifier the wireless device50 can detect, such as a Cell ID, SS, etc. This implementation approachallows the transmission point 30 to flexibly change the number ofsequences it uses, e.g., responsive to changing its beamformingconfiguration.

The wireless device 50 in some embodiments may be configured to receive,and the network 20 configured to send, detailed configurationinformation regarding the synchronization signals to be used by thewireless device 50 for synchronization and/or radio measurements. Forexample, the wireless device 50 can be informed of whether thesynchronization signals in question are transmitted periodically oraperiodically. In the case that they are periodic, the wireless device50 can be informed of the periodicity regarding subframes, frames, OFDMsymbols, or any other time measure known by the wireless device 50. Inthe case that the synchronization signals are aperiodic, the wirelessdevice 50 can be configured with some mechanism to trigger theirtransmission, or to be told when they are transmitted. The wirelessdevice 50 can also be informed about the time/frequency resources thesesets are transmitted. For example, the resource elements might beexpected in the center of a given frequency carrier or other parts ofthe spectrum.

In some embodiments, the wireless device 50 may be updated with a newset of synchronization signals to use for source synchronization. Theupdate can be triggered by the network or self-triggered by the device,such as upon the detection of a new identifier with stronger radioconditions or by an update in system information configuration withinthe same coverage area.

These updates may be triggered in at least two scenarios. The firstscenario occurs when the network 20 decides to use a different set ofsignals (e.g., MRSs). That can occur in the case when the network 20decides that, for the transmission point 30 transmitting for theinvolved wireless device 50, more synchronization signals will bedefined and transmitted. For instance, more synchronization signals aredefined in order to transmit narrower beams from a time T0 to a time T1.

If more synchronization signals are transmitted, but the previous onesare still being transmitted, the network 20 may either update thewireless device 50 or not update the wireless device 50. In other words,if the transmission point 30 serving as the synchronization source forthe wireless device 50 reconfigures its antenna beamforming to useadditional beams and begins transmitting additional synchronizationsignals for the added beams but continues transmitting the priorsynchronization signals, the network 20 does not necessarily need totell the wireless device 50 about the added synchronization signals.

The network 20 may also decide to reduce the number of synchronizationsignals in a given transmission point. In that case, the wireless device50 can be informed so that it does not trigger failure procedures. Thesecond scenario occurs when the wireless device 50 is in Idle mode andis configured to obtain synchronization with a given set ofsynchronization signals. In that case, mobility is device-based, i.e.,an autonomous procedure. Therefore, the update of the set ofsynchronization signals to be considered as a synchronization source canbe done autonomously by the wireless device 50, based, for example, onthe radio quality associated with received synchronization signals. Itis also possible that the synchronization signals encode some notion ofgrouping that the wireless device 50 can detect.

The network 20 may also decide to update a configuration of the existingsynchronization signals, such as the repetition per synchronizationsignal, the number of synchronization signals, the time/frequencyresources, the periodicity at which the beam sweeps occur (i.e. DTXperiod between sweeps), etc.

In some embodiments, the network 20 can increase the periodicity forwhich the synchronization signals are transmitted, based on the factthat there is no data being scheduled to at least a subset of wirelessdevices 50. This approach can be applicable in the case that aDemodulation Reference Signal, DMRS, or other signals on downlinkcontrol channels, e.g., the Physical Downlink Control Channel, PDCCH, orthe Packet Data Channel, PDCH, in the NR context, carry theirsynchronization signals, because in such cases wireless devices 50 canobtain synchronization as long as downlink data transmissions are beingscheduled.

In some embodiments, the network 20 may adapt synchronization signaltransmission as a function of the carrier frequencies or frequency bandsinvolved. For example, the network 20 may be configured to transmitsynchronization signals more often when higher carrier frequencies arebeing used, as compared to when, relatively speaking, lower carrierfrequencies are being used. There may be a defined configurationparameter used by network nodes to determine whether or when to increasethe frequency or repetition. In some cases, the wireless device 50 maybe updated with a new set of synchronization signals even when thenetwork is not changing its set.

The synchronization source may be updated during mobility. When awireless device 50 knows its synchronization signal set, such as definedby a set of MRSs, the device 50 can distinguish these MRSs from otherMRSs that do not belong to its set and treat the other MRSs as neighborMRSs. The network 20 can use the neighbor MRSs as a new synchronizationreference to be used by the wireless device 50 during a mobilityexecution procedure. The wireless device 50 uses the MRS indicated bythe network as a synchronization source reference to send a PRACHpreamble to another transmission point 30.

A second scenario where the update of the synchronization source, e.g.,set of MRSs, may occur is when the wireless device 50 performs amobility operation. Assuming that the wireless device 50 is configuredwith a set of MRSs denoted as MRS1, MRS2, MRS3, the device 50 mayreceive a handover command from its source transmission point 30indicating that the device should use one or potentially multiple otherMRSs, e.g. MRS4, MRS5, MRS6, as source synchronization signals orsynchronization reference before a PRACH preamble is transmitted. In thecase where a single MRS is indicated in the handover command, whichcould be in a Radio Resource Control, RRC, Connection Re-configurationmessage, the device 50 can be further updated by the new source accessnode with a set of MRSs to be used as the new synchronization source.Otherwise, in the case where multiple MRSs are given in the handovercommand, there could be an indication that the wireless device 50 shouldassume these as the new synchronization source.

Various embodiments described herein target the usage by wirelessdevices 50 in the Connected mode where the devices 50 can use one ormultiple sets of synchronization sequences as their synchronizationsources. However, some embodiments may be used in the Idle mode, or anykind of sleeping state where a wireless device 50 uses synchronizationfor initial access and paging monitoring. In other words, the device 50could be moving across the coverage of one or multiple synchronizationsignals and not consider it as a “reselection” or consider the samesystem information parameters, e.g., PRACH configuration as stillapplicable. The configuration in that case will either occur viadedicated signaling when the device 50 was Connected or via systeminformation.

In addition to ultra-lean qualities and beamforming, there may bedecoupling between Idle and Connected mode connectivity. The decouplingmay involve some transmission points 30 being configured to support only“RRC Connected” wireless devices 50. That is, some transmission points30 should not be primarily used for initial access or device-basedmobility. Similarly, other transmission points 30 may be configured toonly support “RRC Idle” wireless devices 50 or both “RRC Idle” and “RRCConnected” wireless devices. Such configurations will affect the kind ofsignals and identifiers these nodes will transmit in these differentconfigurations. “Idle” in this context refers to the RRC Idle state butthe meaning should be understood as extending to any kind of sleepingstate where a wireless device 50 is optimized for battery savings. InLTE for example, Idle comprises procedures such as Suspend/Resume.

Assuming a decoupling between Idle and Connected mode connectivity asdescribed, a wireless device 50 configured with a set of MRSs should notassume that this is the same synchronization signal to be used in Idlemode unless it is configured. In other words, a wireless device 50 issynchronized via a set of MRSs and moves to Idle mode. In that state,the wireless device 50 should search for new signals to obtainsynchronization where the new signal(s) can also be one or multiplesignals, according to some of the techniques described above.

In LTE, a base station broadcasts a pair of sequences, e.g., PSS andSSS, in omnidirectional fashion, for use as a synchronization source.However, it is contemplated herein that a transmission point 30transmits two or more synchronization signals on a beam-specific basis,such that the transmission point 30 transmits a synchronization signalcorresponding to each directional beam, among two or more directionalbeams used by the transmission point 30 in beamforming. Any one or moresuch synchronization signals are, therefore, valid synchronizationsequences with respect to the transmission point 30, and a wirelessdevice 50 maintains synchronization so long as it receives at least onethem with sufficiently good reception.

An advantage of the some of the techniques described above is that theusage of multiple sets of synchronization sequences/MRSs as thesynchronization source means that a wireless device 50 does not need toupdate its synchronization source every time it detects a stronger beam.Also, the usage of multiple synchronization signals from a transmissionpoint 30 simplifies the actions taken by a wireless device 50 upondetecting radio link problems, since it allows different implementationsfor quickly recovering lost synchronization.

FIG. 12 illustrates an example functional module or circuit architectureas may be implemented in a transmission point 30. The illustratedembodiment at least functionally includes a transmitting module 1202 anda processing module 1204. The processing module 1204 is configured totransmit, the the transmitting module 1202, the two or moresynchronization signals from the transmission point 30 on abeam-specific basis, such that each synchronization signal correspondsto a respective one among two or more directional beams used by thetransmission point (30) in antenna beamforming. The synchronizationsignals serve as references for synchronization measurements by wirelessdevices 50, for obtaining or maintaining synchronization with thetransmission point 30.

In some embodiments the processing module 1204 is configured to generatetwo or more synchronization signals, each synchronization signal beingdistinguishable from the other one or ones of the two or moresynchronization signals. Further, the processing module 1204 isconfigured to transmit, the the transmitting module 1202, the two ormore synchronization signals from the transmission point 30 on abeam-specific basis, such that each synchronization signal correspondsto a respective one among two or more directional beams used by thetransmission point (30) in antenna beamforming. The synchronizationsignals serve as references for synchronization measurements by wirelessdevices 50, for obtaining or maintaining synchronization with thetransmission point 30.

In some embodiments the processing module 1204 is configured to generatetwo or more synchronization signals wherein each synchronization signalbelongs to a set of synchronization signals associated with thetransmission point. Further, the processing module 1204 is configured totransmit, the the transmitting module 1202, the two or moresynchronization signals from the transmission point 30 on abeam-specific basis, such that each synchronization signal correspondsto a respective one among two or more directional beams used by thetransmission point (30) in antenna beamforming. The synchronizationsignals serve as references for synchronization measurements by wirelessdevices 50, for obtaining or maintaining synchronization with thetransmission point 30

FIG. 13 illustrates an example functional module or circuit architectureas may be implemented in a wireless device 50. The illustratedembodiment at least functionally includes a receiving module 1302configured for receiving signals from transmission points 30 in awireless communication network 20, along with a processing module 1304configured for determining a set of synchronization signals used by atransmission point 30. In some embodiments each synchronization signalin the set may be associated with a different directional beam used bythe transmission point 30 in antenna beamforming, and the transmissionpoint 30 uses two or more directional beams. The processing module 1304is further configured for maintaining synchronization with thetransmission point (30) in conjunction with moving between coverageareas corresponding to the two or more directional beams, based ondynamically synchronizing or resynchronizing with detected ones of thesynchronization signals in the set. In this way the wireless device mayautonomously maintain synchronization with the transmission point 30.

With the above examples in mind, in some contemplated embodiments, theavailability of multiple sets of synchronization sequences/MRSs allowsfor different implementations, for a wireless device 50 to regainsynchronization with its source. The procedure may be triggered when thewireless device 50 detects a radio link problem, such as when it is notable to decode downlink control channels.

In some embodiments, re-synchronization may occur in response to thedetection of a radio link problem. Assuming that the wireless device 50has been configured with a set of MRSs (or any other set ofsynchronization signals) to be used as the synchronization source, thewireless device 50 can use the configured resources in different ways.

In some embodiments, the wireless device 50 may detect a radio linkproblem when it is not able to decode downlink control channels, such asPDDCH and PDCH as described in the context of NR. The detection can bedone by counting the number of out-of-sync events similarly to LTE. Inan example case where the wireless device 50 has been configured withMRS1, MRS2, and MRS3, any of the three can be used as a synchronizationsource from the network perspective. In one embodiment, upon reaching acertain number (N310-nr) of out-of-sync packets, the wireless device 50tries to regain synchronization with its previous source access node,randomly selecting one of the configured MRSs (MRS1, MRS2 or MRS3). Inanother embodiment, the wireless device 50 may use the first out of theconfigured set of MRSs that the wireless device 50 can detect. Inanother embodiment, the wireless device 50 selects the strongest MRSthat it has measured out of MRS1, MRS2, and MRS3 and starts to count thenumber of in-sync packets. If the number is not increasing, and at thesame time the number of out-of-sync continues to grow, the wirelessdevice 50 can use the second strongest MRS out of MRS1, MRS2, and MRS3.

The wireless device 50 may also use the multiple MRSs in a smart way toavoid the overhead of detecting/measuring multiple MRSs. In oneembodiment, especially applicable in the case of analogue beamforming(BF), MRSs are transmitted in different beams in a time multiplexedmanner. This approach allows the wireless device 50 to refrain fromdecoding the rest of MRSs for synchronization as long as it detects oneof the MRSs in a good enough manner. Here, “good enough” may be definedby a threshold. In other words, the wireless device 50 may only use onesynchronization signal and, as long as this is above a certainthreshold, it does not need to use the others so that processing can besaved. On the other hand, the wireless device 50 may start trying todetect the others in the case of dropped quality, movements, etc.

Notably, modifications and other embodiments of the disclosedinvention(s) will come to mind to one skilled in the art having thebenefit of the teachings presented in the foregoing descriptions and theassociated drawings. Therefore, it is to be understood that theinvention(s) is/are not to be limited to the specific embodimentsdisclosed and that modifications and other embodiments are intended tobe included within the scope of this disclosure. Although specific termsmay be employed herein, they are used in a generic and descriptive senseonly and not for purposes of limitation.

In some examples, an RRC CONNECTED NR UE may perform RRM measurementsand RRC driven mobility using these measurements. In some instances, anRRC CONNECTED NR UE does not need to be informed about ‘cells’, butrather only about beams. For RRM Measurements in NR an LTE UE detectscells based on its PSS/SSS. An important property of these signals isthat a neighbor cell to be detected does not need to be in-sync with theserving cell's signals. Secondly, the UE autonomously detects theneighbor cell IDs (PCI) from an acquired PSS/SSS, i.e., the network doesnot need to provide a neighbor cell list. UEs typically detect andmeasure neighbor cells by sampling a short time window (e.g., 5 ms) onthe target frequency (which may be the same or different from serving)and search (possibly offline) for PSS/SSS occurrences within thatsample. For each detected PSS/SSS, the UE can also perform a measurementusing the CRS corresponding to the PCI. The result of that action is alist of neighbor cell IDs and corresponding measurement sample. The NRCONNECTED mode operation can rely substantially on beam forming. Besidesthe data- and control information, the reference signals also need to bebeam-formed in order to enable a UE to detect, measure and report beams(rather than cells). One option would be to send both, cell- andbeam-specific sync- and reference signals. If the latter are dependenton the former, a UE would first have to detect the cell (as in LTE)based on its PSS/SSS like signals and subsequently attempt to detect oneor more dependent beam reference signals (BRS). However, if the cellspecific PSS/SSS is not beam-formed while the BRS is, the difference inreceived power will make the simultaneous reception challenging for theUE. Alternatively, the eNB could send the same cell specific PSS/SSSrepeatedly with a number of different BRSs. However, this would increasethe overhead and make it more difficult to use the PSS/SSS asunambiguous timing reference.

In order to keep the UE complexity for detecting beams equally simple asdetecting neighbor cells in LTE, the cell specific synchronizationsignals (PSS/SSS) may be replaced by beam specific synchronizationsignals. These signals should have similar properties to the PSS/SSSwith the primary difference being that they are only used in CONNECTEDmode, and that their time/frequency allocation is not hard-coded in thestandard. As indicated by the name, the beam-specific reference signalsare intended to be beam-formed, and the network could use the increasedallocation flexibility to stagger multiple signals within a subframe(e.g., one per OFDM symbol) and to transmit in different frequencyallocations. The ID revealed by this sync signal is a “beam ID” ratherthan a cell ID. The UE should be able to perform RRM measurements onthese signals and, hence, the signals are denoted Mobility ReferenceSignal (MRS).

Whether several MRSs are beam-formed within a sector or whether a singleMRS covers the entire sector depends on the network configuration and istransparent to the UE. For NR CONNECTED mode, cell specific cell andreference signals are replaced by beam-specific mobility referencesignals. An RRC CONNECTED UE detects and measures individual beams basedon these measurement reference signals. Even though a UE will typicallydetect several such MRSs originating from the same transmission point,there may be potential benefits in allowing the UE to identify groups ofbeams and possibly define it as a “cell”. In some examples an RRCCONNECTED UE might not identify groups of beams.

In LTE, the RRCConnectionReconfiguration with mobilityControlInfocomprises, in particular, the target cell ID. In order to execute thehandover, the UE shall detect the PSS/SSS carrying that PCI andestablish downlink sync with that signal. Due to beamforming, thecoverage area of the synchronization signals becomes potentially smallercompared to the coverage area of a cell. An RRC based mobility upon eachbeam change should be avoided. Mobility across the beams originatingfrom one transmission point and among the beams of tightly synchronizedtransmission points of the same network node should not require any RRCreconfigurations. To achieve this, the network configures the UE with aset of serving MRSs. If the UE's MRS-search reveals several MRS IDslisted in the “serving MRS set”, it chooses the strongest one as timingreference. Provided that the transmitted MRSs are in tight sync, thenetwork does not need to know which of the MRSs in the set the UE usesinstantaneously.

Upon connection establishment and during RRC level mobility, the networkconfigures the NR UE with a “set of serving MRSs” that are transmittedin tight synchronization and among which the UE may use any as timingreference. The UE is be able to distinguish beams from its serving andneighbor eNBs, e.g., to trigger mobility events and measurement reports.The serving MRSs can be used for that purpose so that every beam that isnot in its serving MRS set is a neighbor MRS. The MRSs are not the onlysignal based on which the UE may maintain sync with the network. Whilethe PSS/SSS-like MRS enables the UE to acquire initial sync, thedemodulation reference signals (DM-RS) allow a UE to maintain accuratesync while receiving data. This is similar to LTE, where UEs may, e.g.,use CRS to maintain sync even in between the PSS/SSS occasions. Inaddition to the “set of serving MRSs”, the UE may use its DMRS formaintaining accurate time/frequency sync. In LTE, all physical channelsare scrambled with the cell ID (which the UE acquired from the PSS/SSS).This scrambling ensures that UEs can distinguish transmissions of theserving cell from transmissions of the neighbor cells. Furthermore, thedifferent scrambling sequences randomize the neighbor cells'interference. Since the MRS is beam specific, and since the chosen MRSwithin the “serving MRS set” should be transparent to the network, theMRS ID cannot be used for that scrambling. MRSs are only intended foroperation in CONNECTED mode where the UE should operate in accordancewith a dedicated RRC configuration. Hence, the scrambling ID to be usedby RRC Connected UEs can be conveyed by dedicated signalling rather thanderived from a synchronization signal. In some examples the scramblingID to be used by RRC Connected UEs is conveyed by dedicated RRCsignalling.

1-37. (canceled)
 38. A method implemented by a transmission pointconfigured for operation in a wireless communication network, the methodcomprising: transmitting two or more synchronization signals from thetransmission point on a beam-specific basis, such that eachsynchronization signal corresponds to a respective one among two or morebeams used by the transmission point in antenna beamforming, wherein thetwo or more synchronization signals serve as references forsynchronization measurements by wireless devices, for obtaining ormaintaining synchronization with the transmission point.
 39. The methodof claim 38, further comprising transmitting information from thetransmission point to enable the wireless device to determine the two ormore synchronization signals as belonging to a set of synchronizationsignals.
 40. The method of claim 38, further comprising generating thetwo or more synchronization signals wherein the generating includesdifferentiating the two or more synchronization signals in terms ofincluded information or signal properties, thereby enabling a receivingwireless device to distinguish between the two or more synchronizationsignals.
 41. The method of claim 38, wherein the two or moresynchronization signals comprise a set of Mobility Reference Signals(MRSs), each MRS comprising a Time Synchronization Signal (TSS) and aBeam Reference Signal (BRS), wherein each BRS is unique within the setof MRSs, and wherein each MRS is associated with a different one of thetwo or more beams used by the transmission point in antenna beamforming.42. The method of claim 38, further comprising adapting the transmissionof at least one synchronization signal for the corresponding beam independence on at least one of: radio link conditions between thetransmission point and one or more wireless devices that are operatingin a coverage area of the corresponding beam; and a monitoredsynchronization quality of one or more wireless devices operating in thecoverage area of the corresponding beam.
 43. The method of claim 38,further comprising, for one or more of the beams, dynamicallydetermining at the transmission point which downlink resources to usefor transmitting the corresponding synchronization signal.
 44. Themethod of claim 38, further comprising transmitting assistanceinformation from the transmission point, the assistance informationidentifying the two or more synchronization signals or otherwiseproviding information needed for wireless devices to detect or identifythe two or more synchronization signals.
 45. The method of claim 38,further comprising dynamically changing the number of beams used by thetransmission point in antenna beamforming and correspondingly changingthe number of synchronization signals in use by the transmission point,such that the transmission point transmits a different synchronizationsignal for each beam.
 46. A transmission point configured for operationin a wireless communication network, the transmission point comprising:transceiver circuitry; and processing circuitry configured to: transmit,via the transceiver circuitry, two or more synchronization signals fromthe transmission point on a beam-specific basis, such that eachsynchronization signal corresponds to a respective one among two or morebeams used by the transmission point in antenna beamforming; wherein thetwo or more synchronization signals serve as references forsynchronization measurements by wireless devices, for obtaining ormaintaining synchronization with the transmission point.
 47. Thetransmission point of claim 46, wherein the processing circuitry isconfigured to transmit information from the transmission point, toenable the wireless device to determine the two or more synchronizationsignals as belonging to a set of synchronization signals.
 48. Thetransmission point of claim 46, wherein the processing circuitry isconfigured to differentiate the two or more synchronization signals interms of included information or signal properties, thereby enabling areceiving wireless device to distinguish between the two or moresynchronization signals.
 49. The transmission point of claim 46, whereinthe two or more synchronization signals comprise a set of MobilityReference Signals (MRSs), each MRS comprising a Time SynchronizationSignal (TSS) and a Beam Reference Signal (BRS), wherein each BRS isunique within the set of MRSs, and wherein each MRS is associated with adifferent one of the two or more beams used by the transmission point inantenna beamforming.
 50. The transmission point of claim 46, wherein theprocessing circuitry is configured to adapt the transmission of at leastone synchronization signal for the corresponding beam in dependence onat least one of: radio link conditions between the transmission pointand one or more wireless devices that are operating in a coverage areaof the corresponding beam; and a monitored synchronization quality ofone or more wireless devices operating in the coverage area of thecorresponding beam.
 51. The transmission point of claim 46, wherein theprocessing circuitry is configured to, for one or more of the beams,dynamically determine which downlink resources to use for transmittingthe corresponding synchronization signal.
 52. The transmission point ofclaim 46, wherein the processing circuitry is configured to dynamicallychange the number of beams used by the transmission point in antennabeamforming and correspondingly change the number of synchronizationsignals in use by the transmission point, such that the transmissionpoint transmits a different synchronization signal for each beam.
 53. Amethod of operation by a wireless device configured for operation in awireless communication network, the method comprising: determining a setof synchronization signals used by a transmission point in the wirelesscommunication network; and maintaining synchronization with thetransmission point in conjunction with moving between coverage areascorresponding to two or more beams, based on detected ones of thesynchronization signals in the set.
 54. The method of claim 53, whereindetermining the set of synchronization signals comprises receivingassistance information that identifies the set of synchronizationsignals or provides information enabling the wireless device to identifythe set of synchronization signals.
 55. The method of claim 53, whereinthe transmission point comprises a first one of neighboring first andsecond transmission points in the wireless communication network, andwherein the method further comprises detecting one or moresynchronization signals comprising a set determined to be associatedwith the second transmission point, and changing over from using thefirst transmission point as the synchronization source for the wirelessdevice to using the second transmission point as the synchronizationsource for the wireless device, based on determining that a radioquality determined by the wireless device for one or moresynchronization signals detected from the second transmission point ishigher than a radio quality determined by the wireless device for anydetected synchronization signal from the first transmission point. 56.The method of claim 53, further comprising determining, based onreceiving information from the wireless communication network, thedownlink resources used for transmitting each synchronization signal forthe corresponding beam.
 57. The method of claim 53, wherein the set ofsynchronization signals comprises a set of Mobility Reference Signals(MRSs), each MRS comprising a Time Synchronization Signal (TSS) and aBeam Reference Signal (BRS), wherein each BRS is unique within the setof MRSs, and wherein each MRS is associated with a different one of thetwo or more beams used by the transmission point in antenna beamforming.58. The method of claim 53, wherein maintaining synchronization with thetransmission point comprises, in instances where two or more of thesynchronization signals in the set of synchronization signals aredetected by the wireless device, selecting a strongest orhighest-quality one of the two or more detected synchronization signals,for use in maintaining synchronization with the transmission point. 59.A wireless device configured for operation in a wireless communicationnetwork, the wireless device comprising: transceiver circuitryconfigured for receiving signals from transmission points in thewireless communication network; and processing circuitry operativelyassociated with the transceiver circuitry and configured to: determine aset of synchronization signals used by a transmission point; andmaintain synchronization with the transmission point in conjunction withmoving between coverage areas corresponding to two or more beams, basedon dynamically synchronizing or resynchronizing with detected ones ofthe synchronization signals in the set.
 60. The wireless device of claim59, wherein the processing circuitry is configured to determine the setof synchronization signals based on receiving assistance informationthat identifies the set of synchronization signals or providesinformation enabling the wireless device to identify the set ofsynchronization signals.
 61. The wireless device of claim 59, whereinthe transmission point comprises a first one of neighboring first andsecond transmission points in the wireless communication network, andwherein the processing circuitry is configured to detect one or moresynchronization signals comprising a set determined to be associatedwith the second transmission point, and change over from using the firsttransmission point as the synchronization source for the wireless deviceto using the second transmission point as the synchronization source forthe wireless device, based on determining that a radio qualitydetermined by the wireless device for one or more synchronizationsignals detected from the second transmission point is higher than aradio quality determined by the wireless device for any detectedsynchronization signal from the first transmission point.
 62. Thewireless device of claim 59, wherein the processing circuitry isconfigured to determine, based on receiving information from thewireless communication network, the downlink resources used fortransmitting each synchronization signal for the corresponding beam. 63.The wireless device of claim 59, wherein the set of synchronizationsignals comprises a set of Mobility Reference Signals (MRSs), each MRScomprising a Time Synchronization Signal (TSS) and a Beam ReferenceSignal (BRS), wherein each BRS is unique within the set of MRSs, andwherein each MRS is associated with a different one of the two or morebeams used by the transmission point in antenna beamforming.
 64. Thewireless device of claim 59, wherein the processing circuitry isconfigured to, in instances where two or more of the synchronizationsignals in the set of synchronization signals are detected by thewireless device, select a strongest or highest-quality one of the two ormore detected synchronization signals, for use in maintainingsynchronization with the transmission point.